1. Technical Field of the Invention
The present invention relates to a process for the injection molding of fluid-filled plastic bodies and an apparatus to carry out this process.
2. Discussion of the Related Art
A process of this kind is already known from West German patent application DE-05 21 06 546. In this process, a mold cavity for a shoe heel is formed by a two-part mold provided with an ejector punch. First, this cavity is filled partially with plastic melt by means of a nozzle attached to an injection unit to result in a flowable plastic melt. Next, a pressurized fluid--preferably compressed air--is blown by means of a second nozzle, arranged separately from the first, in such a manner into the fluid center of the plastic already located in the mold cavity that the plastic is pressed universally and uniformly against the walls of the mold cavity. The second nozzle is arranged in that half of the mold bearing it in such a manner that it is connected stationarily to it, always points in the direction of the opening and closing motion of the mold, and always attains the fluid center of the plastic when the mold is closed with its mold cavity-sided outlet When the mold is opened, the opening left by the second nozzle in the shoe heel results in a pressure balance between the interior and exterior of the shoe heel. The goal of this process is to economize plastic material and thus minimize the weight of the final product without any impairment to the stability of the final product.
Another process of this kind is disclosed in U.S. Pat. No. 4,101,617, in which the flowable plastic melt and the pressurized fluid--for example air, carbon dioxide or nitrogen--is introduced into the mold cavity by means of a coaxial combination of nozzles. This combination comprises a central nozzle having a circular cross section for the pressurized fluid and an annular nozzle which envelopes the central nozzle for the flowable plastic melt, both running into a joint opening in the mold. In one embodiment, only one part of the whole quantity of the plastic required for the final product is injected into the mold cavity and then the fluid is injected together with the rest of the required plastic. In another embodiment, the plastic and fluid are injected in separate stages. The disclosed result matches that of the aforementioned DE-05 21 06 546. The generated plastic hollow bodies are, for example, double windows, transparent hollow bricks, double walled lighting fixtures and double walled boarder lights. The pressure between the interior and exterior of the plastic hollow bodies i equalized by withdrawing the coaxial combination of nozzles from the opening of the mold before the mold is opened to remove the final product. Alternatively, the pressure is equalized by sealing the gas inlet opening of the hollow body after the body is formed and cooled by pushing a plug-forming quantity of plastic in and boring or piercing the finished hollow body after the mold has been partially or totally opened. The mold cavity can be designed either as unchangeable during the injection molding cycle or as variable during such a cycle by means of at least one suitable lifting punch in the mold.
A process that is similar to the one above is also known from the West German publication DE-PS 28 00 482. This process has the major difference of using a viscous liquid rather than a ga as the fluid to produce a cavity in a plastic hollow body.
Another process of this kind is known from British patent GB-PS 2 139 548, in which a fluid is blown by means of one or more nozzles, which are separated from the nozzle which injects the flowable plastic melt, into the plasticized plastic flowing into the mold cavity. The fluid nozzle or nozzles empties or empty into a runner in the mold and/or also at a suitable point or at suitable points in the actual mold cavity. After the plastic body has cooled in the mold cavity and before the mold is opened, the pressure between the interior--comprising, if the occasion arises, several individual cells--of this plastic body and its exterior is equalized by means of the nozzle(s) installed to introduce the fluid.
In each of these aforementioned processes, only as much plasticized plastic as is necessary to shape the final product is injected into the mold cavity and the fluid must be blown in, whether this blowing takes place simultaneously with the introduction of the flowable plastic melt or later, so long as the flowable plastic melt exhibits initial signs of cooling on the parts of the mold surface with the melt has already made contact. Evidently, in the case of geometrically simple bodies this does not lead to difficulties when fabricating final products with repeatably uniform quality. However, in the case of geometrically complicated bodies with different cross sectional areas vertical to the flow direction of the flowable plastic melt in the mold cavity, for example, in the case of a plate provided with hollow reinforcing ribs on one side, different effects which prevent manufacture of final products with repeatably uniform quality can be expected with the aforementioned methods.
It is to be expected that the flowable plastic melt in the mold cavity, both before and during the blowing of a fluid, flows faster into regions whose cross sections are larger than into regions whose cross sections are smaller and that this effect occurs to a greater degree when a fluid is blown in. Thus, when a fluid is blown in, the flowable plastic melts flows generally into regions whose cross section are greater not only before but rather simultaneously sideways into the adjacent regions whose cross sections are smaller; and in the extreme case a partial reversal in the flow direction of the flowable plastic melt can occur upon reaching the outer wall in conjunction with a break through of the fluid through the outer skin of the plastic body. In any event, disturbing flow marks are produced on the surface of the final product. Only in very special individual cases can this be avoided by fixing the filling picture beforehand, i.e., chronologically changing the behavior of the flow front of the flowable melt and taking the picture into consideration when constructing the mold and, thus, the mold cavity for the geometrically complicated final product.
In addition, for a plastic body with a largely solid construction and only a few, relatively narrow fluid-filled cavities, such a pressure or follow-up pressure must be generated in the entire mold cavity just by means of the pressure of the fluid in these cavities and of the plastic material to be forced out of the cavities so that, following cooling the fluid-filled plastic body exhibits no sink marks on its surface. This, too, can be achieved only in special individual cases when the fluid is prevented from simultaneously breaking through the outer skin of the final product or at least weak points are prevented from occurring in the fabricated parts.
In addition, in the case of final products whose cross sections vary widely or which have special shapes such as curved pipes, only in individual cases can it be repeatedly predetermined what the temperature gradient will be at every individual injection molding cycle, e.g., at a specific cross section through the mold cavity during the simultaneous formation of the wall of the still flowable plastic body and its fluid-filled cavity. However, the actual position of the cavity cross section in the body cross section, for example that of the pipe interior within the pipe body in the region of the pipe curvature, is predefined since the longitudinal axis of a cavity under discussion agrees in essence with the line of the respectively highest temperature of the flowable plastic melt in the flow direction, provided that additional influences of friction and current mechanics do not also have to be considered. A curved pipe manufactured by one of the aforementioned methods can thus exhibit in a cross section in the region of the pipe curvature from one injection molding cycle to another different positions of the pipe inner wall relative to the axis of the pipe and the automatically rotationally symmetrical pipe outer wall and thus have a different wall thickness up to the break-through on the periphery of the pipe.
Therefore, it is an object of the present invention to provide a process and a suitable device to carry out the process wherein fluid-filled plastic bodies having a complicated geometric shape can also be manufactured while avoiding the aforementioned drawbacks and having a faultless surface, in particular without flow marks apart from eventually provided inlet and outlet openings o their subsequent seals.
It is a further object to manufacture bodies having a few, relatively narrow fluid-filled cavities in a largely solid plastic body which show no sink marks in the surface.
Also, it is yet another object to enclose the fluid-filled cavities at predetermined points within the plastic body and with essentially repeatable volume.
Further objects and advantages are apparent from the specification and drawing which follow.